The modification of membrane components, mainly proteins that occur in cells transformed by oncornaviruses are studied in order to ascertain the role of such modifications in alterations of cell properties that are expressed at or mediated by the cell surface. Analysis is made of the role of the viral glycoproteins in the behavior of transformed cells and its interaction with specific cellular receptors. Cell membranes are isolated and their composition determined by electrophoresis and immunological procedures. Avian myeloblastosis virus glycoprotein is isolated and its structure determined by physicochemical methods. Different cleavage products are used to characterize the glycoprotein cellular receptor in target cells.